1. Field of the Invention
The present invention is directed to the measurement of airborne particles and aerosols through condensational growth.
2. Description of the Related Art
Airborne particles are ever present in the environment. Microscopic particles in the air include soil, smoke, photochemical, salt, dusts, fumes, mists, smog, and atmospheric water or ice particles. The presence of these particulates affects visibility, climate, health and quality of life. These airborne particles are examples of aerosols. Aerosols are generally defined as solid or liquid particles suspended in a gas.
Many measurement methods for aerosol particles rely on condensational growth to enlarge particles to a size that can be detected by optical or other means. Condensational growth is also used to enable the collection of particles for chemical analysis. One type of particle measurement device is commonly referred to as a condensation particle counter (CPC). CPCs specifically examine the number concentration of particles that increase in size by condensational growth. This growth results from supersaturation of a condensing vapor in the surrounding gas. The saturation ratio is defined as the partial pressure of a vapor over its saturation vapor pressure. The saturation vapor pressure is the pressure required to maintain a vapor in mass equilibrium with the condensed vapor (liquid or solid) at a specified temperature. Supersaturation refers to that portion of the saturation ratio greater than 1.0.
According to this method, particles grow using a supersaturated vapor to a sufficiently large size for easy detection and quantification by optical methods. The aerosol is first exposed to the vapor of a working fluid (such as butanol, alcohol, or water) in a saturation chamber. Subsequently, vapor condensation onto particles is induced by either adiabatic expansion or cooling in the condensing chamber, or by mixing with a cooler airflow. The formed droplets are then detected using light scattering or attenuation techniques.
CPCs suffer from two general issues: low flow rates and the use of toxic chemicals as working fluids.
The earliest detectors saturated an air sample with water vapor, and then expanded the air adiabatically to produce cooling and subsequent condensation onto the particles. (J. Aitken: On the number of dust particles in the atmosphere, Proc. Royal Soc. Edinburgh 35, 1888).
An automated condensation particle counter using this principal was disclosed in U.S. Pat. No. 2,684,008. This was a semi-continuous instrument that cycled between the sample and expansion modes. Another design, disclosed in U.S. Pat. No. 3,694,085, shows an automatic, semi-continuous counter that used mixing to cool and condense.
Continuous, laminar flow condensation particle counters pass the sample air flow through a saturator and then through a condenser. The saturator mixes the air with a condensable vapor such as butanol. From the saturator the air passes into a condenser tube that is cooler than the saturator. The cooling of the airflow within the condenser creates a supersaturation region and results in condensational growth of the suspended particles such that they can be counted optically. (See, J. Bricard, P. Delattre, G. Madelaine and M. Pourprix in Fine Particles, B. Y. H. Liu, editor, Academic Press, NY, 1976, pp 565-580; U.S. Pat. No. 3,806,248). This approach has been used extensively for particle number concentration measurement. Many have refined the method through use of a plurality of streams, improved saturator design, or temperature control.
Current continuous, laminar flow particle condensation instruments use cooled-wall condensers. The devices create supersaturation because, in part, the thermal diffusivity of the gas is greater than the mass diffusivity of the condensing vapor. Condensation is achieved by cooling the flow such that the temperature drops more quickly than the condensing fluid can diffuse, thereby creating a region of supersaturation. Particles within this supersaturation region will grow by condensation. These systems do not work well for particles suspended in air when the condensing fluid is water. With water the degree of supersaturation achieved is small because the water vapor diffuses too quickly, before the temperature of the sample stream is lowered.
Hence, these systems are typically operated with butanol, which has a vapor mass diffusivity of 0.081 cm2/s. (The mass diffusivity for water vapor is more than three times higher, 0.265 cm2/s.) The thermal diffusivity of air, which determines the rate of heat transfer, is 0.215 cm2/s.
Yet, for many applications, it is desirable to use water as the condensing fluid. Water is nontoxic and inexpensive. Water-based condensation counters would be suitable for measurements in offices, homes and other inhabited locations. They present less of a problem for operation in clean rooms, such as those used for microchip manufacture. Water is preferred over butanol or other fluids when collecting particles for chemical analysis.
Early counters used water as the condensing substance, but were not continuous. (See, J. Aitken: On the number of dust particles in the atmosphere, Proc. Royal Soc. Edinburgh 35, 1888; U.S. Pat. Nos. 2,684,008 and 3,694,085). Alternative designs, such as that disclosed in U.S. Pat. No. 4,449,816, show a continuous condensation counter that may be used with water based on the mixing of two saturated fluids with differing temperatures. Yet another design, shown in U.S. Pat. No. 6,330,060, discloses a continuous flow cloud condensation nucleus counter that employs a segmented condenser, with alternating hot and cold rings to Droduce well-controlled, albeit low, sudersaturation (See also, W. A. Hoppel, S. Twomey and T. A. Wojchiechowski (J. Aerosol Sci 10: 369-373, 1979).
Hence, a continuous flow device having a high flow rate and using non-toxic chemicals would be useful.
The present invention, roughly described, pertains to a method for enlarging particles by condensation. The method may be utilized in the detection, counting or other analysis of particles in aerosols. The method includes the steps of: introducing a particle-laden flow at a first temperature; and passing the flow through a condenser having a second temperature greater than the flow and a vapor pressure of a condensing vapor at walls of the condenser near saturation. In a further aspect, the condensing fluid is water.
In another embodiment, the invention is a method comprising the steps of: forming a particulate sample at a first temperature; and passing the particulate sample through a wet walled chamber including interior walls provided at a second temperature greater than the first temperature, and wherein a condensing fluid is near its saturation vapor pressure at the walls.
In yet another embodiment, the invention is a particle condensation apparatus. The apparatus includes an inlet receiving an aerosol flow, and a preconditioner coupled to the inlet and having a first temperature, the preconditioner having an outlet. A condenser is coupled to the outlet of the preconditioner and receives the aerosol flow from the preconditioner. The condenser has interior walls provided at a second temperature higher than the first temperature. This difference in temperature may be achieved by cooling the flow in the preconditioner or by heating the condenser, or by a combination of both. In a further aspect, the condenser is tubular in shape. In yet another aspect, the condensing vapor in the apparatus has a vapor pressure at the interior walls which is near saturation.
In another embodiment, the invention comprises a particle condensation apparatus. The apparatus includes a sample inlet receiving a particle laden airflow having a first temperature, and a condenser having interior walls provided at a second temperature higher than the first temperature and having a wet surface. In unique embodiments, the second temperature is 15xc2x0 C. or greater than the first temperature, 25xc2x0 C. or greater than the first temperature and 45xc2x0 C. or greater than the first temperature.
These and other objects and advantages of the present invention will appear more clearly from the following description in which the preferred embodiment of the invention has been set forth in conjunction with the drawings.